Mass spectrometer method and tube for equalizing contact potentials of tube electrodes during operation thereof by direct heating



March 24, 1970 H. GENTSCH 7 3,502,868

MASS SPECTROMETER METHOD AND TUBE FOR EQUALIZING CONTACT POTENTIALS OF TUBE ELECTRODES DURING OPERATION v THEREOF BY DIRECT HEATING Filed Sept. 2'7. 196'? FIG.I.

ATTORNEYS United States Patent L Int. Cl. B01d 59/ 46; H01j 39/36 US. Cl. 250-413 13 Claims ABSTRACT OF THE DISCLOSURE A mass spectrometer tube, which operates according to the omegatron principle, having apparatus for approximately uniformly heating all its electrodes. The apparatus is operative, for example, to bake out platinum-iridium electrodes prior to the process of measurement at about 600 C., and maintain the electrodes, during the process of measurement, at 150 C.

BACKGROUND OF THE INVENTION The present invention relates to a mass spectrometer tube which operates according to the omegatron principle.

Various types of mass spectrometer tubes, operating according to the omegatron principle, are known in the art. Various improvements of the electrode configuration have been made in later known types of tubes to make the high frequency field homogenous and, therefore, to improve the conductive effect via-a-vis the ions to be identified. Further improvements have been aimed at a reduction in the space charge which is produced by nonresonant ions.

These improved mass spectrometer tubes, in comparison with corresponding types in older apparatus, show an increased mass resolving power and greater accuracy in the determination of the relative abundance of gas components in a gas mixture. The test data obtained with these improved tubes, however, is still not sufficiently reproducible. More particularly, it is not possible to obtain the necessary stability of the calibration curve for the measurement of partial pressures in gaseous mixtures.

SUMMARY OF THE INVENTION An object of the present invention is to improve the reproducibility of measurement results in mass spectrometer tubes which operate according to the omegatron principle.

This and other objects, which will become apparent in the discussion that follows, are achieved according to the present invention by a heating apparatus arranged in connection with a mass spectrometer tube and, in particular, with the tube electrode system. The apparatus is designed to at least approximately uniformly heat all the electrode elements during the process of measurement.

After considerably improving the electrode systems which influence the field structure in a mass spectrometer,

as explained above, it was discovered that the cause of the varying values of the calibration curve had to involve the gas adsorption of the electrodes and the consequential change of the contact potential. Since, however, a homogeneous change in the contact potential on all the electrode surfaces could not explain the insufficient stability of the calibration curve, it was assumed that, in addition to the well known microscopic heterogeneity of the electrode surfaces, a macroscopic heterogeneity of the contact potentials must also occur on the electrode surfaces.

It is possible to explain the variations in the calibration curve which have occurred in the past by the presence of a heterogeneous or nonuniform surface potential. The corresponding variations in the contact potential are caused by the adsorption of gases in the electrodes with the resulting occurrence of strong inhomogeneous electrostatic fields along the surface of the electrodes.

Proceeding from such a mental conception, which, however, in view of the complicated physical processes which occur in connection with the surface effects, is somewhat simplified, the goal of the invention is to reduce the surface absorption by uniformly heating the electrode system during the process of measurement. Heating permits the adsorbed molecules to diffuse into the free space of the gas container and results in a reduction of the contact potential gradients along the electrode surfaces. Whereas, as has been described, a uniform change of the surface potential does not impair the stability of the calibration curve, a non-uniform change does; it therefore appears essential that the electrode system be heated in such a way that all the electrodes of the system be at the same temperature.

A temperature between and 200 C., preferably of about C., is sufficient to activate the surface diffusion of practically all the adsorbed gases for electrode systems made of platinum-iridium. It may also be advantageous to bake out the electrodes at a temperature of 600 C.; for polished platinum-iridium electrodes, this latter temperature requires electrical heating power of approximately 1 watt per square centimeter of electrode surface.

BRIEF DESCRIPTION OF THE DRAWINGS FIGURE 1 is a perspective view of one electrode of a mass spectrometer tube having a heating element in accordance with one embodiment of the present invention. FIGURE 2 is a schematic and representational diagram of apparatus for the controlled heating of an electrode according to another embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, FIGURE 1' shows a resistance heating element 2 attached to one electrode 1 of the electrode system of a mass spectrometer tube. This heating element 2 may be constructed either as a double stranded winding of heating wire or as a strip of heat conductor laid upon a foundation of insulating material. The heating element may be attached to an electrode by any suitable means, as, for example, by clamps 3.

In another embodiment of the invention, shown in FIGURE 2, the heating element for the electrode 1 is constructed as a jacketed thermocouple 4. This embodiment makes possible the simultaneous use of thermocouples, one for each electrode, both for heating and for measuring the temperature of the different electrodes.

The thermocouple 4 shown in FIGURE 2 is alternately connected to an AC. power source 5 and a temperature measuring unit 6, of a type well known in the art. The circuit 7, which is shown with a simple switch but should be understood to include electronic (e.g., transistor) means, connects the thermocouple 4 to the AC. power source 5 during half the current wave; then, during the time span of the next successive half wave,'to the DC. supply of the temperature measuring unit 6. By feeding back the resulting measured temperature values to control the amount of heating power fed to the thermoelement, either by hand or electronically, for example, by means of the thyristor device shown, the electrode system may be maintained at any preselected temperature.

A mass spectrometer tube equipped with the apparatus which forms the present invention ofiers technical advantages of considerable importance. The calibration curve of this mass spectrometer tube shows an unusual temporal stability without subsequent adjustment of the electrical operating parameters, such as the high frequency and the DC. voltages of the electrodes. It becomes unnecessary to externally inductively heat the mass spectrometer tube, a costly process by means of which the tube has to be removed from the magnet. Because of the uniform heating of the electrode system during the process of measuring, the speed of release of the adsorbed gases is so great that the disturbances which are known in the art as memory effects practically no longer appear.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations.

I claim:

1. In the method of mass spectrometry which employs a spectrometer tube operating according to the omegatron principle, the improvement comprising the step of substantially equalizing the contact potentials of the surfaces of the electrodes of said tube by directly heating each of said electrodes during operation thereof so as to maintain them at a uniform and constant temperature.

2. In a mass spectrometer tube which operates according to the omegatron principle and has an electrode system with a plurality of electrodes, th improvement comprising heating means arranged immediately adjacent all of said electrodes for directly heating substantially uniformly each of said electrodes during the process of measurement, so as to substantially equalize the contact potential of the electrode surfaces.

3. The improvement defined in claim 2 wherein said heating means are resistance heating elements.

4. The improvement defined in claim 3 wherein said resistance heating elements are double stranded windings.

5. The improvement defined in claim 2 wherein said heating means are jacketed thermocouples.

6. The improvement according to claim 2 further comprising means for measuring the temperature of said electrodes and control means connected to said measuring means and said heating means for maintaining the temperature of said heating means at a preselected value.

7. The improvement defined in claim 6 wherein said measuring means comprises one or more resistance thermometers.

8. The improvement defined in claim 6 wherein said measuring means comprises one or more thermocouples.

9. The improvement defined in claim 6 wherein said measuring means includes sensing elements and said heating means includes heating elements, at least some of said elements being arranged as both heating and sensing elements.

10. The improvement defined in claim 9 wherein said sensing elements are thermocouples.

11. The improvement defined in claim 2 wherein said heating means is arranged to maintain the temperature of said electrodes in the range of 100-200 C.

12. The improvement defined in claim 11 wherein said heating means is arranged to maintain the temperature of said electrodes at approximately 150 C.

13. The improvement defined in claim 11 wherein said heating means is additionally arranged to bake out said electrodes prior to said process of measurement at a temperature in the range of 400800 C. 1

References Cited UNITED STATES PATENTS 2,511,981 6/1950 Hanchett. 2,954,470 9/1960 Brashear. 3,204,462 9/1965 Horne 73-359 FOREIGN PATENTS 887,891 1/1962 Great Britain. 961,019 6/ 1964 Great Britain. 968,912 9/1964 Great Britain. 1,218,182 4/1964 Germany.

RALPH G. NILSON, Primary Examiner C. E. CHURCH, Assistant Examiner 

